FIG. 1 schematically illustrates a traditional approach for dimming a light source 2. A variable resistor 3 is connected in series with the light source 2, and this series arrangement is connected to an input 1, which is to be connected to mains, which can be considered to be a source of constant amplitude alternating voltage. As a result, the light source will receive less voltage. Disadvantages of this approach include the dissipation occurring in the resistor, the reduced accuracy of the dimming level at low intensities, and a color shift of the light output due to changing LED current.
FIG. 2 schematically illustrates a different but nevertheless traditional approach for dimming a light source 2. A dimmer 4 receives the AC voltage from mains at its input, and outputs a phase-cut AC voltage, i.e. a voltage that is substantially zero for a range of phases and substantially equal to the input voltage for the remaining phases. A disadvantage of such approach is that it may lead to visible flicker and stroboscopic effects.
FIG. 3 schematically illustrates an approach which avoids the above disadvantages, and which will be indicated as “duty cycle dimming at higher frequency”. A converter 5 receives the AC mains voltage, and outputs a DC current. A chopper 6 receives the DC current from the converter 5, and outputs a chopped current having a chopping frequency typically in the range of 300 Hz to 3 kHz, at least higher than the mains frequency. “Chopping” means that in a first portion of the current period (i.e. the inverse of the chopping frequency) the current is zero while in a second portion of the current period the current is equal to the received input current, or vice versa. The ratio of the duration of the second portion to the entire period is indicated as the duty cycle; varying the duty cycle varies the average current.